There is no telling what they had in stock or on order at the end of July '69. Minimum order size is a factor; if for example, each tach had a 1000 pc order minimum, the 6/7 was slower moving and more likely to have an inventory surplus. When '69 production was extended, they probably were not certain how long it would last.
The July/August 1996 Camaro Enthusiast had a feature on a '69 Z/28 project, built during the last week of production. Had the 6500/8000 tach, 1112003 distributor and black-face Nova gauges.